1. Field of the Invention
The present invention relates to a vehicle control system for controlling a vehicle of its own to follow a preceding vehicle while running in pursuit.
2. Description of the Prior Art
One known vehicle control system is disclosed in Japanese laid-open patent publication No. 6-144076. The disclosed vehicle control system controls a vehicle of its own based on the distance between the vehicle of its own and a preceding vehicle so as to follow the preceding vehicle.
The vehicle control system has a vehicle-to-vehicle distance detector for detecting the distance from its own vehicle which incorporates the vehicle control system to a preceding vehicle, a steering angle detector for detecting a steering angle of its own vehicle, a vehicle speed detector for detecting a vehicle speed of its own vehicle, a turn radius calculator for calculating the radius of a turn which its own vehicle is making based on the detected steering angle and the detected vehicle speed, an evaluator for evaluating the probability that the preceding vehicle is running in the lane of its own vehicle based on the calculated turn radius and the detected vehicle-to-vehicle distance, and a controller for controlling the throttle valve or brake of its own vehicle based on the evaluated probability.
In operation, the vehicle control system calculates the radius of a turn which its own vehicle is making based on the detected steering angle and the detected vehicle speed, and decides whether the preceding vehicle is present in the lane of its own vehicle based on the calculated turn radius and the detected vehicle-to-vehicle distance.
The vehicle control system quantitatively evaluates the possibility of an erroneous detection of a preceding vehicle based on the fact that such an erroneous detection depends on the turn radius and the vehicle-to-vehicle distance, and adjusts a control gain for the throttle valve or brake control process depending on the possibility of the erroneous detection, i.e., the probability that the detected preceding vehicle is positioned in the lane of its own vehicle, for thereby varying the characteristics of its own vehicle with which to follow the preceding vehicle.
Since the control gain is lowered with respect to a preceding vehicle whose possibility of being erroneously detected is high, the vehicle control system will not be adversely effected by a preceding vehicle running in an adjacent lane.
The disclosure of Japanese laid-open patent publication No. 6-144076 is silent about how the vehicle control system operates when its own vehicle changes lanes.
When the driver of its own vehicle turns the steering wheel clockwise to move its own vehicle from the present lane to a right lane, the conventional vehicle control system determines a front vehicle running on the right side as a new preceding vehicle to follow. At this time, when the driver returns the steering wheel counterclockwise to orient its own vehicle along the right lane, the conventional vehicle control system tends to determine a vehicle running in the original lane, from which its own vehicle has moved to the right lane, as a new preceding vehicle to follow.
Conversely, when the driver of the vehicle turns the steering wheel counterclockwise to move its own vehicle from the present lane to a left lane, the conventional vehicle control system determines a front vehicle on the left side as a new preceding vehicle to follow. At this time, when the driver returns the steering wheel clockwise to orient its own vehicle along the left lane, the conventional vehicle control system tends to determine a vehicle running in the original lane, from which the vehicle has moved to the left lane, as a new preceding vehicle to follow.
Specifically, such a problem will be described below with reference to FIGS. 1A-1C and 2 of the accompanying drawings.
It is assumed that a vehicle Z of its own, i.e., a vehicle which incorporates the vehicle control system, follows a preceding vehicle P while running along a lane in a road RT, as shown in FIG. 1A, and then changes from the lane to an adjacent lane across a lane line CL, as shown in FIG. 1B, and after changing to the adjacent lane, its own vehicle Z follows a preceding vehicle Q in the adjacent lane, as shown in FIG. 1C.
FIG. 2 of the accompanying drawings shows the manner in which the steering angle .theta. of the vehicle Z varies when the vehicle Z changes the lanes. In FIG. 2, the steering angle .theta. is positive when the steering wheel of the vehicle Z is turned counterclockwise, and negative when the steering wheel of the vehicle Z is turned clockwise. The yaw rate of the vehicle Z is plotted in the same pattern as shown in FIG. 2 when the vehicle Z changes the lanes.
The steering angle .theta. goes through different values a-e shown in FIG. 2 as the vehicle Z moves through successive corresponding points a-e shown in FIG. 1B while changing the lanes.
If a preceding vehicle is determined based on the yaw rate or the steering angle of the vehicle Z without detecting the lane change, then the preceding vehicle may erroneously be determined when the steering wheel is returned in the vicinity of the point d.
Specifically, while the vehicle Z is running along the original lane, it locks on and follows the preceding vehicle P which is also running in the same lane. When the vehicle Z changes from the original lane to a right lane, it unlocks the preceding vehicle P, and moves into the right lane by turning the steering wheel clockwise. Immediately before the vehicle Z completes the lane change, the steering wheel of the vehicle Z is returned counterclockwise in the vicinity of the point p, and the vehicle Z tends to recognize the vehicle P as a preceding vehicle again.
Therefore, until the vehicle Z completes the lane change, the vehicle Z cannot determine the vehicle Q definitely as a preceding vehicle. As a result, a relatively long period of time is spent from the time when the vehicle Z unlocks the preceding vehicle P to the time when the vehicle Z finally locks on the preceding vehicle Q. For this reason, the driver of the vehicle Z is likely to feel that the vehicle Z is not making the lane change smoothly.